Mainline GPUI2 UI work (#3099)

This PR mainlines the current state of new GPUI2-based UI from the
`gpui2-ui` branch.

Included in this is a performance improvement to make use of the
`TextLayoutCache` when calling `layout` for `Text` elements.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/storybook/src/stories/components/multi_buffer.rs

@@ -0,0 +1,24 @@
+use ui::prelude::*;
+use ui::{hello_world_rust_buffer_example, MultiBuffer};
+
+use crate::story::Story;
+
+#[derive(Element, Default)]
+pub struct MultiBufferStory {}
+
+impl MultiBufferStory {
+    fn render<V: 'static>(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        let theme = theme(cx);
+
+        Story::container(cx)
+            .child(Story::title_for::<_, MultiBuffer<V>>(cx))
+            .child(Story::label(cx, "Default"))
+            .child(MultiBuffer::new(vec![
+                hello_world_rust_buffer_example(&theme),
+                hello_world_rust_buffer_example(&theme),
+                hello_world_rust_buffer_example(&theme),
+                hello_world_rust_buffer_example(&theme),
+                hello_world_rust_buffer_example(&theme),
+            ]))
+    }
+}